1. Field of the Invention
This invention relates to drive circuits for providing conduction state control of inductively-loaded switching devices such as power transistors. More particularly, the invention relates to direct base drive circuits for power switching transistors in the inverter sections of DC-DC and DC-AC converter circuits.
2. Description of the Prior Art
Conduction control, or base-drive, circuits of the prior art generally fall within one of two classes--direct drive or indirect drive arrangements. Direct drive arrangements use conduction control elements directly coupled to the base of the power switch under control, while indirect drive arrangements use an intermediate transformer to couple the control circuitry to the base circuit of the power switch. Regardless of the direct or indirect coupling used, such base drive circuits of the prior art have been found to suffer from the problem of inefficient bias power drain during normal operation cycles. The increased bias drain results from base drive circuitry wherein at least one conduction controlling device is ON during the entire interval of conduction or non-conduction of the power switching device being controlled.
Known relevant examples of the prior art are shown in U.S. Pat. Nos. 3,610,963--Higgins, 3,657,569--Froeschle, 3,930,170--Burens et al., 3,983,418--Wallace, and 3,986,052--Hunter. Each of these references disclose bias drain through a switching control device after the initiation of one of the two power device conduction states, resulting in inefficient power loss during the normal course of operation of the base drive circuits.
In U.S. Pat. Nos. 3,610,963--Higgins, control transistor 10 of FIG. 1 is held conductive for the duration of the OFF state of controlled power transistor 38 causing DC bias drain from source 22 for the entire duration of the OFF state of transistor 38.
3,657,569--Froeschle discloses that control transistor Q1 of the single drawing FIGURE is conductive for the entire duration of the ON state of controlled transistor Q2 causing DC bias drain from source 21.
Similarly, 3,930,170--Burens et al. shows control transistor 10 of FIG. 1 conductive for the entire duration of the OFF state of controlled transistor 36, thus causing DC bias drain from source 72. Inefficient DC power loss can also be shown at control transistor Q2 of FIG. 1 of 3,983,418--Wallace and at control transistor 102 of FIG. 1 of 3,986,052--Hunter.
In addition to bias power losses, direct drive arrangements of the prior art suffer from current limitations in the base circuit of the device being controlled and require the addition of a large power diode to the emitter circuit of the controlled device to provide sufficient reverse base-emitter potential for effecting carrier sweep-out when turn-off is initiated.